Unshielded twisted-pair (UTP) cables are becoming the preferred transmission medium for local area networks (LANs). The LAN industry is strongly interested in solutions that permit the use of UTP cables for token ring (TR) systems currently requiring shielded twisted pair cables (STP).
In TR systems, the distance between a wiring concentrator and TR stations attached to it is typically limited to 100 m. Therefore, the maximum cable length between two successive stations is 200 m. The attenuation of UTP cables is in the range of 10 . . . 13 dB/100 m at 16 MHz, where the spectral peak of Manchester-coded signals occurs. For the systems considered, it is assumed that cable attenuation varies between 0 and 25 dB at 16 MHz.
To solve the equalization problem, it has been proposed to use fixed filters for compromise equalization. However, for minimal and maximal cable lengths, significant residual signal distortion will still occur with this approach and degrade the noise margin for signal detection. Insufficient equalization also increases timing jitter due to variations of the zero crossings in the received signal.
It has also been suggested to use line build-out equalizers, cf. A. Rosenblatt, "Technology '91: Data Communications", IEEE Spectrum, pp. 48-51, Vol. 28, No. 1, Jan. 1991. These equalizers can be understood as a combination of a variable-length cable simulator and an appropriate fixed filter, located at the receiver front-end of a data transmission system. A single control parameter is adjusted such that a given output signal level is obtained. The combined lengths of the actual cable and the simulated cable add up to a known cable length for which the overall channel is equalized.